New York — Despite polls that predict a runaway Reagan victory, Northeastern Democrats are hunkering down for a last few days of campaigning that some insist could produce a ''Truman-like'' upset for presidential candidate Walter Mondale.
In New York State, where these Democrats feel the Mondale-Ferraro slate has the best chance, a planned campaign visit to Buffalo by Mr. Mondale last night and a scheduled joint appearance with Geraldine Ferraro in New York City today is aimed at strengthening support.
''We are not giving up at all,'' says James P. Keane, a Buffalo city councilor and Mondale delegate at the Democratic convention. ''We are pressing forward.''
Mr. Keane sees a ''smashing victory'' for Mondale in his traditionally Democratic city, and adds that the ticket is leading in all of Erie County.
Added to strong showings in New York City, Keane expects Mondale will have enough to carry New York State, which, with its bonanza of 36 electoral votes, is second only to California.
''I still feel pretty positive,'' says Hope L. Mason, a planner in New York City. Like many Democrats, she expresses skepticism over predictions for Tuesday.
''The polls are superficial,'' says Ms. Mason, whose stint as a Jackson delegate at the convention was her first real experience in politics.
Katie Davis, a hospital administrator in Brooklyn and another Jackson delegate, suggests that pollsters do not bother to question many of the ''average people on the street'' who will vote for Mondale, particularly those from poorer areas.
But not all Democrats are as hopeful. Lawrence Mandelker of Katonah predicted a US landslide victory for Reagan in September, and he still holds to that view.
''My sense is that (Mondale) will take New York City and that he has a decent chance to take the state,'' says Mr. Mandelker, a lawyer and a Hart delegate this summer.
Long Island, which is predominantly Republican, will go for Reagan, Mandelker says. But if Mondale can win the large cities and suburban Westchester County, he could win here, the delegate adds.
And in Milford, Conn., Mayor Alberta Jagoe says most Democrats in the area feel that Reagan will be easily reelected.
Mayor Jagoe is more involved in local races, including a close race between incumbent Bruce A. Morrison (D) and Lawrence J. DeNardis (R), who lost the Third District seat to Mr. Morrison in 1982.
She says Morrison's polls show Reagan leading, but that voters favor Morrison.
''People do tend to split tickets here,'' says Jagoe.''I'm hoping they go for (Morrison).''
A Hart delegate at the Democratic convention, Jagoe has not done much campaigning for the Mondale-Ferraro ticket, adding that there hasn't been a large effort in her community by the campaign staff.
Issues are still being touted as the key to a good showing by Democrats.
In Buffalo, City Councilor Keane says the President's leadership and the ''fairness'' issue are being discussed as the area's depressed economy continues to lag.
In Brooklyn, Ms. Davis has debated a black Republican, bringing up the issue of domestic spending vs. the military budget; Reagan's educational record and cuts in college loans for students; and civil rights vs. the conservative platform adopted by the GOP at its national convention in Dallas.
Robert P. Zimmerman, a marketing executive on Long Island, has also been debating on behalf of the Democratic ticket.
''Once you get beyond the personalities and focus on particular issues, it does generate a response,'' says Mr. Zimmerman, a Mondale delegate.
He says President Reagan's ''slick'' strategy in visiting a Jewish synagogue on Long Island last week did have an impact.
But he says Jewish voters are also worried about constitutional and religious issues.
It will be a ''rough night'' on Long Island, Zimmerman says, but he hasn't thrown his hands up in the air.
''The polls almost supercede why the candidates are running,'' he says.
He is also disappointed in the emphasis on personalities in this election.
''Right now we have a very charismatic President, a 'proclaimed' recovery, and peace,'' he says.
It is difficult, he adds, to project the military problems the country may have in several years or the effect the large deficits will have in two years.
Now Democrats are concentrating on getting the vote out on Tuesday.
''We are stressing - in churches, at gatherings - that our people must go to the polls,'' says Ms. Davis of Brooklyn.
She tells black voters that they cannot make the mistake they did in 1980, not bothering to vote because a Reagan victory was predicted.
In many states, the vote was close, and if blacks had been registered and had voted, President Carter might have won, she says.